Radiometric measuring arrangements are usually applied when conventional fill level measuring devices are not applicable, due to special requirements at the measuring location. For instance, very frequently, extremely high temperatures and pressures reign at the measuring location, or chemically and/or mechanically very aggressive, environmental influences are present, which make the use of other measuring methods impossible.
Especially in the case of very high containers, measuring arrangements are frequently used, wherein two or more radioactive radiators are placed externally on a side of the container, one above the other, in order to cover the entire measurable height of the container with radioactive radiation.
Used as radiators are e.g. Co 60 or Cs 137 preparations placed in a radiation protection container. The radiation protection containers have an opening, through which the radiation emitted by the radiator escapes. A radiation direction is selected, such that the radiation penetrates those regions of the container to be registered for the measurements. In the case of a plurality of radiators installed one above the other, the exit openings are preferably directed in such a manner, that the sum of the resulting radiation cones covers the total measuring range as uniformly as possible.
On the oppositely lying side, the radiation intensity emerging from the container is quantitatively registered with a detector. The radiation intensity depends on geometric arrangement and absorption. The latter depends on the amount of fill substance in the path of the radiation in the container. As a result, total radiation intensity detected by the detector is a measure for current fill level of fill substance in the container.
A suitable detector is e.g. a scintillation detector equipped with a rod-shaped, solid scintillator and an optoelectrical transducer, such as e.g. a photomultiplier. Gamma radiation is converted by the scintillation material into light flashes, which are registered by the photomultiplier and converted into electrical pulses. The pulse rate, with which the pulses occur, depends on the total radiation intensity impinging on the detector and, thus, is a measure for the fill level.
The detector includes, as a rule, an electronics, which makes available to a superordinated unit an output signal corresponding to the pulse rate. The electronics comprises usually a control system and a counter. The electrical pulses are counted and a counting rate derived, on the basis of which fill level is ascertained.
There are, however, a number of applications, in which, e.g. due to very high pressures occurring in the container, very thick-walled containers must be applied. If one would apply, in such case, radiators located in radiation protection containers outside of the container, the radiation would have to pass through two thick container walls on the path from the radiator to the detector. In order that, in this case, a radiation intensity sufficient for the fill level measurement can arrive at the detector, radiators with very high activities, or very high energy isotopes, must be applied, such as cobalt, for instance. High activities are, however, undesirable, for reasons of radiation protection. High energy isotopes have, as a rule, a markedly smaller half life and must, accordingly, be replaced more often.
Instead, the radiators are preferably placed in pressure resistant, protective tubes inserted laterally through bores in the container wall. The radiators are located, therewith, in the interior of the container, so that their radiation need penetrate only one of the two thick container walls on the path to the detector. In this way, the radiative power required for fill level measurement is markedly reduced, and correspondingly weaker radioactive sources can be applied.
In the case of two or more radiators provided in this way, one above the other, in the container, it has been found, however, that, at fill levels around the installation height of the radiator, an extremely non-linear dependence of the measured radiation intensity on fill level is obtained. The reason for this is that the radiators located in the container send radioactive radiation in all directions. An individual radiator not covered by fill substance radiates into regions both above and below its installed height. If, now, the fill level rises sufficiently that the radiator is covered by fill substance, then the entire radiation of the radiator, or at least a very large part thereof, is absorbed by the fill substance. If one starts with an initially empty container, which is filled continuously, then the total radiation intensity falling on the detector sinks first continuously with increasing fill level. As soon as the fill level, however, exceeds the installed height of the radiator, there arises an extremely non-linear dependence of the measured radiation intensity on fill level, since the fill substance now not only absorbs the radiative power of this radiator radiated laterally and downwards but also almost the entire radiative power of this radiator radiated upwardly. A very small changing of the fill level leads, thus, in the case of fill levels in the regions of the installed height of the respective radiator to a very large change in the measured radiation intensity. In this way, there results an extremely non-linear dependence of the total detected radiation intensity on fill level.